The field of the present invention relates to purification of organic compounds, and more particularly to the purification of the compound (20R)-19-nor-24-difluoro-1α, 25-dihydroxyvitamin D3 (referred to herein as “24F2-DM”) by preparing the compound in crystalline form.
Purification of organic compounds, especially those designated for pharmaceutical use, is of considerable importance for chemists synthesizing such compounds. Preparation of the compound usually requires many synthetic steps and, therefore, the final product can be contaminated not only with side-products derived from the last synthetic step of the procedure but also with compounds that were formed in previous steps. Even chromatographic purification, which is a very efficient but relatively time-consuming process, does not usually provide compounds which are sufficiently pure to be used as drugs.
Depending on the method used to synthesize 1α-hydroxyvitamin D compounds, different minor undesirable compounds can accompany the final product. Thus, for example, if direct C-1 hydroxylation of the 5,6-trans geometric isomer of vitamin D is performed, followed by SeO2/NMO oxidation and photochemical irradiation, (see Andrews et al., J. Org. Chem. 51, 1635 (1986); Calverley et al., Tetrahedron 43, 4609 (1987); Choudry et al., J. Org. Chem. 58, 1496 (1993)), the final 1α-hydroxyvitamin D product can be contaminated with 1β-hydroxy- as well as 5,6-trans isomers. If the method consists of C-1 allylic oxidation of the 4-phenyl-1,2,4-triazoline-3,5-dione adduct of the pre-vitamin D compound, followed by cycloreversion of the modified adduct under basic conditions, (see Nevinckx et al., Tetrahedron 47, 9419 (1991); Vanmaele et al., Tetrahedron 41, 141 (1985) and 40, 1179 (1994); Vanmaele et al., Tetrahedron Lett. 23. 995 (1982)), one can expect that the desired 1α-hydroxyvitamin can be contaminated with the pre-vitamin 5(10), 6,8-triene and 1β-hydroxy isomer. One of the most useful C-1 hydroxylation methods, of very broad scope and numerous applications, is the experimentally simple procedure elaborated by Paaren et al., J. Org. Chem. 45, 3253 (1980); and Proc. Natl. Acad. Sci. U.S.A. 75, 2080 (1978). This method consists of allylic oxidation of 3,5-cyclovitamin D derivatives, readily obtained from the buffered solvolysis of vitamin D tosylates, with SeO2/t-BuOOH and subsequent acid-catalyzed cycloreversion to the desired 1α-hydroxy compounds. Taking into account this synthetic path it is reasonable to assume that the final product can be contaminated with the 1α-hydroxy epimer, the 5,6-trans isomer and the pre-vitamin D form. 1α-hydroxyvitamin D4 is another undesirable contaminant found in 1α-hydroxyvitamin D compounds synthesized from vitamin D2 or from ergosterol. 1α-hydroxyvitamin D4 results from C-1 oxidation of vitamin D4, which in turn is derived from contamination of the commercial ergosterol material. Typically, the final product may contain up to about 1.5% by weight 1α-hydroxyvitamin D4. Thus, a purification technique that would eliminate or substantially reduce the amount of 1α-hydroxyvitamin D4 in the final product to less than about 0.1-0.2% would be highly desirable.
The vitamin D conjugated triene system is not only heat- and light-sensitive but it is also prone to oxidation, leading to the complex mixture of very polar compounds. Oxidation usually happens when a vitamin D compound has been stored for a prolonged time. Other types of processes that can lead to a partial decomposition of vitamin D compounds consist of some water-elimination reactions. The driving force for these reactions is the allylic (1α-) and homoallylic (3β-) position of the hydroxy groups. The presence of such above-mentioned oxidation and elimination products can be easily detected by thin-layer chromatography.
Usually, all 1α-hydroxylatation procedures require at least one chromatographic purification. However, even chromatographically purified 1α-hydroxyvitamin D compounds, although showing consistent spectroscopic data that suggests homogeneity, do not meet the purity criteria required for therapeutic agents that can be orally, parenterally or transdermally administered. Therefore, it is evident that a suitable method of purification of the 1α-hydroxylated vitamin D compound 24F2-DM is required.